Abstract
C4 plants have a biochemical carbon-concentrating mechanism that increases CO2 concentration around Rubisco in the bundle sheath. Under low light, the activity of the carbon-concentrating mechanism generally decreases, associated with an increase in leakiness (ϕ), the ratio of CO2 retrodiffusing from the bundle sheath relative to C4 carboxylation. This increase in ϕ had been theoretically associated with a decrease in biochemical operating efficiency (expressed as ATP cost of gross assimilation, ATP/GA) under low light and, because a proportion of canopy photosynthesis is carried out by shaded leaves, potential productivity losses at field scale. Maize plants were grown under light regimes representing the cycle that leaves undergo in the canopy, whereby younger leaves initially developed under high light and were then re-acclimated to low light (600 to 100 μE·m−2·s−1 photosynthetically active radiation) for 3 weeks. Following re-acclimation, leaves reduced rates of light-respiration and reached a status of lower ϕ, effectively optimizing the limited ATP resources available under low photosynthetically active radiation. Direct estimates of respiration in the light, and ATP production rate, allowed an empirical estimate of ATP production rate relative to gross assimilation to be derived. These values were compared to modelled ATP/GA which was predicted using leakiness as the sole proxy for ATP/GA, and, using a novel comprehensive biochemical model, showing that irrespective of whether leaves are acclimated to very low or high light intensity, the biochemical efficiency of the C4 cycle does not decrease at low photosynthetically active radiation.
Highlights
The C4 pathway of photosynthesis has been attracting increasing interest in recent years for its high crop productivity potential in the face of global warming and population pressure (Friso et al, 2010; Zhu et al, 2010; Covshoff and Hibberd, 2012)
Maize plants were grown under light regimes representing the cycle that leaves undergo in the canopy, whereby younger leaves initially developed under high light and were re-acclimated to low light (600 to 100 μE·m−2·s−1 photosynthetically active radiation) for 3 weeks
had re-acclimated under low light (HLLL) plants had the lowest A under saturating light, but as light decreased between photosynthetically active radiation (PAR) 250 and 0 μE·m−2·s−1 the response approached that of low light (LL) plants
Summary
The C4 pathway of photosynthesis has been attracting increasing interest in recent years for its high crop productivity potential in the face of global warming and population pressure (Friso et al, 2010; Zhu et al, 2010; Covshoff and Hibberd, 2012). C4 photosynthesis evolved from C3 photosynthesis under the environmental pressure of declining ambient CO2 and increasing transpiration demand in semi-arid environments (Griffiths et al., 2013; Osborne and Sack, 2012). Under optimal conditions, characterized by high temperatures and high light intensities, C4 plants have higher photosynthetic rates than C3 plants (Ehleringer and Pearcy, 1983; Pearcy and Ehleringer, 1984) and very high productivity. Maize (Zea mays, L.), a C4 plant of the NADP-malic enzyme (NADP-ME) subtype, is the leading grain production cereal (www.fao.org/statistics)
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